Description

Rifabutin, a rifamycin antibacterial derivative, is the first agent approved and introduced for the prevention of Mycobacterium avium complex (MAC) in AIDS patients. It is also indicated in combination chemotherapy for the prophylaxis and treatment of MAC infections in HIV positive patients and for newly diagnosed and chronic tuberculosis.

Originator

Archifar (Italy)

Uses

Semisynthetic derivative of Rifamycin S that inhibits nucleic acid synthesis. An antibacterial (tuberculostatic).

Indications

Rifabutin (Mycobutin), an antibiotic related to rifampin, shares its mechanism of action, that is, inhibition of RNA polymerase. Rifabutin has significant activity in vitro and in vivo against M. avium-intracellular complex (MAC) isolates from both HIV-infected and non–HIV-infected individuals. It has better activity against MAC organisms than rifampin. Rifabutin is active against M. tuberculosis, including some rifampinresistant strains, such as M.leprae and M.fortuitum. It has a spectrum of activity against gram-positive and gramnegative organisms similar to that of rifampin. The molecular basis for resistance to rifabutin is shared by both rifampin and rifabutin; this explains the virtually complete cross-resistance that occurs between these drugs.

Brand name

Mycobutin (Pharmacia & Upjohn).

Antimicrobial activity

The activity is similar to that of rifampicin, but it is more active against the Mycobacterium avium complex (MIC 0.01–2 mg/L) and several other atypical mycobacteria. It inhibits the replication of human immunodeficiency virus 1 (HIV-1) in concentrations (10 mg/L) that are not toxic to lymphoid cells, but no efficacy on HIV infections has been demonstrated.

Acquired resistance

The frequency of spontaneously resistant mutants in several bacterial species, including M. tuberculosis, M. leprae, Staphylococcus aureus and Chlamydia trachomatis, is somewhat lower than with rifampicin.

Pharmaceutical Applications

Rifabutine; ansamycin. Molecular weight: 847.02.A semisynthetic spiropiperidyl derivative of rifamycin S, available for oral administration. It is slightly soluble in water and soluble in organic solvents.

Biochem/physiol Actions

Rifabutin is an antibiotic; antitumor. Rifabutin interferes with HSP-90 molecular chaperone, enhances ubiquitination and protein degradation, and inactivates bacterial RNA polymerase.

Pharmacology

Rifabutin is well absorbed orally, and peak plasma concentrations are reached in 2 to 3 hours. Because of its lipophilicity, rifabutin achieves a 5- to 10-fold higher concentration in tissues than in plasma. The drug has a half-life range of 16 to 96 hours and is eliminated in urine and bile. Rifabutin appears as effective as rifampin in the treatment of drug-susceptible tuberculosis and is used in the treatment of latent tuberculosis infection either alone or in combination with pyrazinamide. Clinical use of rifabutin has increased in recent years, especially in the treatment of HIV infection. It is a less potent inducer of cytochrome 450 enzymes pathways than rifampin and results in less drug interaction with the protease inhibitors and nonnucleoside reverse transcriptase inhibitors. Rifabutin is therefore commonly substituted for rifampin in the treatment of tuberculosis in HIV-infected patients. Another important use of rifabutin in the HIV-infected population is prevention and treatment of disseminated MAC.

Pharmacokinetics

Oral absorption：12–20% Cmax 300 mg oral ：0.38 mg/L after 3.3 h Plasma half-life：16 h Volume of distribution：9.3 L/kg Plasma protein binding： 85%absorption and distribution Oral absorption is rapid but incomplete, with considerable interpatient variation. It is well distributed, concentrations in many organs being higher than that in plasma. The average concentration in lungs is 6.5 times the simultaneous plasma concentration.Metabolism and excretion Rifabutin is mainly metabolized to the active desacetyl derivative, although several other oxidation products have been detected in urine, where some 10% of the dose is eliminated. About 30–50% of the dose can be recovered from the feces. Elimination from plasma is biphasic, with a terminal half-life of 45 h. The drug is a weak inducer of hepatic enzymes. The rate of metabolism increases, and the plasma area under the concentration–time curve (AUC) declines as the treatment continues.

Clinical Use

Rifabutin, the spiroimidazopiperidyl derivative of rifamycin B was approved in the United States for the prophylaxis of disseminated MAC in AIDS patients on the strength of clinical trials establishing its effectiveness. The activity of rifabutin against MAC organisms greatly exceeds that of rifamycin. This rifamycin derivative is not effective, however, as monotherapy for existing disseminated MAC disease. Rifabutin is a very lipophilic compound with a high affinity for tissues. Its elimination is distribution limited, with a half-life averaging 45 hours (range, 16 69 hours). Approximately 50% of an orally administered dose of rifabutin is absorbed, but the absolute oral bioavailability is only about 20%. Extensive first-pass metabolism and significant biliary excretion of the drug occur, with about 35% and 53% of the orally administered dose excreted, largely as metabolites, in the feces and urine, respectively. Despite its greater potency against M. tuberculosis in vitro, rifabutin is considered inferior to rifampin for the short-term therapy of tuberculosis because of its significantly lower plasma concentrations. Although rifabutin is believed to cause less hepatotoxicityand induction of cytochrome P450 enzymes than rifampin,these properties should be borne in mind when the drug is usedprophylactically. Rifabutin and its metabolites are highly coloredcompounds that can discolor skin, urine, tears, feces, etc.

Side effects

The adverse effects that most frequently result in discontinuation of rifabutin include GI intolerance, rash, and neutropenia. Rifabutin levels will be increased with concurrent administration of fluconazole and clarithromycin, resulting in anterior uveitis, polymyalgia syndrome, and a yellowish-tan discoloration of the skin (pseudojaundice). Other adverse reactions are similar to those of rifampin, such as hepatitis, red-orange discoloration of body fluids, and drug interactions due to effects on the hepatic P450 cytochrome enzyme system.

Drug interactions

Potentially hazardous interactions with other drugs Anti-arrhythmics: metabolism of disopyramide, and propafenone accelerated; concentration of dronedarone reduced. Antibacterials: increased risk of side effects with azithromycin; clarithromycin and other macrolides increase concentration of rifabutin, resulting in increased risk of uveitis - reduce rifabutin dose; reduced concentration of dapsone and clarithromycin. Anticoagulants: reduced anticoagulant effect of coumarins. Antidiabetics: reduced antidiabetic effect of tolbutamide; possibly reduced antidiabetic effect with sulphonylureas. Antiepileptics: reduced concentration of fosphenytoin, phenytoin and carbamazepine. Antifungals: fluconazole, triazoles, posaconazole and voriconazole increase the concentration of rifabutin resulting in increased risk of uveitis - reduce rifabutin dose; rifabutin reduces concentration of posaconazole, voriconazole and itraconazole - increase voriconazole dose, avoid with isavuconazole and itraconazole. Antipsychotics: possibly reduced aripiprazole concentration - increase dose of aripiprazole. Antivirals: atazanavir darunavir, fosamprenavir, saquinavir and tipranavir and possibly nevirapine increase concentration of rifabutin - halve or reduce dose of rifabutin; efavirenz reduces the concentration of rifabutin - increase dose of rifabutin; concentration of both drugs reduced with etravirine; indinavir increases rifabutin concentration - avoid; concentration of indinavir reduced - increase indinavir dose; concentration of elvitegravir reduced and active metabolite of rifabutin increased - reduce dose of rifabutin; concentration of rilpivirine reduced - increase rilpivirine dose to 50mg once daily; ritonavir increases the concentration of rifabutin resulting in increased risk of uveitis - reduce rifabutin dose; concentration of saquinavir reduced and concentration of rifabutin increased - reduce rifabutin dose; concentration of daclatasvir and simeprevir possibly reduced - avoid; avoid with ledipasvir, sofosbuvir and telaprevir. Atovaquone: concentration of atovaquone reduced (possible therapeutic failure of atovaquone). Ciclosporin: possibly reduced ciclosporin levels. Cobicistat: concentration of cobicistat reduced - adjust cobicistat dose. Corticosteroids: reduced level of corticosteroids - double steroid dose. Give as twice daily dosage. Cytotoxics: possibly reduced concentration of axitinib (increase axitinib dose), bosutinib, cabazitaxel, ceritinib, crizotinib, lapatinib, olaparib, panobinostat, ponatinib and vemurafenib - avoid. Guanfacine: concentration of guanfacine possibly reduced - increase dose of guanfacine. Hormone antagonists: concentration of abiraterone possibly reduced - avoid. Ivacaftor: concentration of ivacaftor possibly reduced - avoid. Oestrogens and progestogens: reduced contraceptive effect due to increased metabolism. Sirolimus: reduced sirolimus concentration - avoid. Tacrolimus: possibly reduced tacrolimus trough concentration. Ulipristal: possibly reduced contraceptive effect - avoid.

Metabolism

Rifabutin is rapidly metabolised in the liver by the cytochrome P450 isoenzyme CYP3A4 mainly to active 25-O-deacetyl and 31-hydroxy metabolites. Rifabutin induces its own metabolism resulting in a lower AUC after 4 weeks of continuous treatment than after the first few doses. About 53% of a dose is found in the urine, mainly as metabolites and about 30% of a dose is excreted in the faeces.